I’ve always more or less seen it as mostly just a training issue as a lot of people think their small size makes it unneeded.

Having had five of them from puppies, one having a litter that we kept two from, (Sister’s idea, I prefer cats) none of them were ever really aggressive unless antagonized. Maybe a bit yippey when the doorbell rang or a stranger was in the house, but after about a minute, they calmed down when they saw there was no threat.